The present invention relates generally to spacecraft attitude control, and more particularly to systems and methods for correcting spacecraft thermal distortion pointing errors.
FIG. 1 is a drawing of one embodiment of a communications spacecraft 100, having a plurality of communications antennas 102–108 mounted on the east and west sides of the spacecraft and on the earth deck. To perform its mission, the spacecraft must maintain the payload antennas pointing at their earth coverage regions at all times. As one skilled in the art will appreciate, this is accomplished using an attitude control system that senses the spacecraft attitude using attitude sensors, such as earth sensors, sun sensors, star sensors, gyros and the like, and applies control torques using reaction wheels or thrusters to null the attitude errors. Although this approach maintains high accuracy pointing of the attitude sensors (typically located on the spacecraft earth deck), the antenna pointing can suffer due to spacecraft structure distortions caused by temperature variations that occur as the sun orientation with respect to the spacecraft changes throughout the day and seasonally.
The thermal distortions can have a significant impact on the spacecraft design and performance. For example, roughly one-third of a typical 0.15 degree antenna pointing error (0.05 degrees) may be caused by structure thermal distortions. For a spacecraft with a payload power of 10 kW and 2 degree spot beams (1250 km in diameter), the payload power must be increased by roughly 10% (1000 Watts) to provide the added coverage area to make up for the thermal distortion pointing errors. To meet this higher requirement, the spacecraft mass may be increased by abut 70 kg, and the spacecraft cost may increase by roughly $1.6 M. The increased mass also may increase the launch cost by requiring the use of a more capable launch vehicle.
Prior art systems attempt to reduce the thermal distortion pointing errors using purely open-loop means. The thermal distortion pointing errors are estimated on the ground using analysis tools, such as MSC.NASTRAN developed by MSC.Software Corporation, based on the material thermo-elastic properties and predicted temperature profiles. The pointing error predictions are used to generate spacecraft attitude steering and antenna gimbal commands that correct for the distortion effects. As is true of any open-loop compensation scheme, the accuracy is highly dependent on the modeling accuracy. Thus, what is needed is a system and method that can correct for thermal distortion pointing errors more accurately.